Ecological and ecotoxicological surveys of moderately contaminated floodplain ecosystems in The Netherlands

Floodplains of the rivers Rhine and Meuse in the Netherlands are moderately polluted with heavy metals and toxic
compounds. In these floodplain areas a number of nature rehabilitation programmes are being planned and executed. The
question arises whether or not the contaminants of concern pose a risk to organisms exposed, and, if so, whether this will hinder
the expected ecosystem recovery. This study focuses on the effects of contaminants on aquatic and terrestrial macro-invertebrates
in the field situation. In three shallow lakes in the floodplains of the river Rhine, showing different degrees of contamination,
chemical analyses and laboratory bioassays have been carried out on sediment samples together with intensive
biological field surveys (a so called TRIAD-approach). A nearly identical set-up was chosen to assess 18 terrestrial floodplain
sites. The concentrations of several contaminants exceed generic critical risk levels. Risk assessment models show that actual
effects on top predators cannot be excluded. Species of lower trophic status might be at risk as well. In the aquatic compartments
chemical, ecotoxicological and ecological results point in the same direction. They can be summarised as moderate biological
effects, which can mainly be attributed to the elevated levels of a mixture of priority pollutants (metals and polyaromatic
hydrocarbons). No large differences were found among the three lakes. At terrestrial sites, however, fewer effects were found in
field surveys and bioassays, while the chemical analyses revealed the same levels of pollution as in the lakes study. This
suggests a lower bioavailability of the contaminants than expected. In conclusion, no extreme effects have been found. This
conclusion only partly supports the predictions made by present day-risk assessment models. The advantages of nature
rehabilitation, that is, improvement of ecological quality through habitat diversity, seem to be larger than the disadvantages
of increasing toxic stress by exposing a larger diversity of species to floodplain contaminants. To confirm this statement
rehabilitation programmes should include overall as well as in-depth monitoring studies.